1. Field of the Invention
The present invention relates to a method for performing a random access in a mobile communication system, and more particularly, to an improved method for performing a random access in a mobile communication system.
2. Background of the Related Art
In general, a CDMA mobile communication system is provided with a plurality of base stations BS each for providing radio communication paths with a plurality of mobile station, a plurality of base station controller BSC each for controlling operation of the base stations, a plurality of mobile switching center MSC each for switching call signals from mobile stations, and a home location registration/visitor location registration HLR/VLR for administrating mobility of mobile stations. A general origination call processing procedure, which comes from each mobile stations in a mobile communication system, will be explained with reference to FIG. 1.
Referring to FIG. 1, upon a mobile station is provided with the power, the base station transmits information for a pilot channel, a synchronizing channel, and a broadcasting(call) channel to the mobile station by turns(a1, b1 and c1). When the mobile station issues the origination call, the mobile station requests the base station for a call connection through the access channel(d1). In this instance, the base station allocates a channel according to request of the mobile station within its own cell or sector, for allowing the call connection if the present reverse channel communication capacity is below a preset threshold value, and if the present reverse channel communication capacity exceeds a preset threshold value, the base station allocates no channel for not allowing the call connection(e1). As shown in FIG. 1, procedure thereafter is a generally known procedure of transmission of a preamble between the base station and the mobile station(g1), and transmission of data between the base station and the mobile station(h1). A random access method called as a slotted ALOHA has been used in the IS-95B cellular CDMA mobile communication system. In the Slotted ALOHA random access method, a plurality of slots each having a predetermined slot period (i.g., 200 ms) are allocated over a reverse common channel, and the mobile station which will transmit the origination call starts to transmit a message at a starting point of the slot for performing random access.
FIG. 2 illustrates a timing diagram for explaining a random access method(Slotted ALOHA) in the IS-95B cellular CDMA mobile communication system, where ‘x’ denotes a time point at which a message to be transmitted is generated, and an arrow ‘1’ denotes a time point an actual transmission is performed irrespective of a channel situation. A slot interval is defined to be longer than a maximum length of a message. However, all access channel message are transmitted during one slot. In such as random access method, a size of an access channel slot is denoted as 4+PAM_SZ+MAX_CAP_SZ, and values of the PAM_SZ and the MAX_CAP_SZ are determined by a message broadcasted from the base station. In data transmission, the mobile station always transmits an 1+PAM_SZ preamble, which has a fixed size, before transmission of a 3+MAX_CAP_SZ message. For example, if one frame is a 20 msec, the PAM_SZ value is 2, and, when the MAX_CAP_SZ has a value 1, an size of an entire access channel slots may be 140 msec. In the aforementioned related art random access method, the base station can receive data transmitted from one mobile station through a slot. If two mobile stations compete for the same slot over a reverse common channel, the base station receives only one signal of the two. If more than two mobile stations compete for the same slot over the reverse common channel, the base station may receive one of the signals. In the random access method shown in FIG. 2, if a transmission message is generated at a position X1 forward of a slot starting time point, the transmission message can be transmitted at a starting point of the slot without waiting time. However, if the transmission message is generated at a position ‘X2’ passed through the starting point of the slot, transmission of the message may be waited until the starting point ‘X3’ of the next slot. If another transmission message is generated during waiting for the starting point ‘X3’ of the next slot, there will be a contention problem in which the two kind of messages collide to each other. Such an access method is applicable to the IS-95B cellular CDMA mobile communication system which has not so many mobile stations and a simple system without any significant problem.
However, the aforementioned related art random access method may not be applied to a mobile communication system developed after the IS-95C which is based on multimedia information such as audio, video, image, text and the like, because it is expected that mobile communication system developed after the IS-95C has much more subscribers registered therein compared to the mobile communication system based on the IS-95B. Data will be multimedia information such as audio, video, image and text having a variable length depending on data characteristics. Therefore, if the random access is performed in this environment according to the related art random access, collision of the transmission messages is vulnerable.
And, when the mobile station is performing random access using the fixed sizes 3+MAX_CAP_SZ of slots, the transmission efficiency may be lower. That is, because even one message which has a message amount smaller than one slot size(for example, 80 msec or 40 msec) occupies one slot completely, an unused portion of the slot is wasted. Those disadvantages provides a cause to a lower a transmission efficiency and, thereby deteriorating quality of service in communication when many mobile stations perform random access during a almost similar time.